In every mod/nomod you play there are easy songs, that give a lot of pp, be it HD, DT, HR.. Those songs are considered free pp/farm. Those songs help you get that boost, but after a while, pp rain stops, as you have scores in all the free pp maps in your current skill, and by playing them you have not improved to get to the next skill level's pp maps. So people who claim that DT = always free pp, have not yet hit the brick wall of no improvement, but eventually they will. DT doesn't make game harder, just a little different - the patterns are easier to read, but the maps are faster in bpm and ar, so transferring from nomod to DT to play at the same level should not be a problem if your nomod play is solid (different with HR though). I'd say, you should be able to play most of DT maps that would give 170-180pp without big problems, if you can't, well, that means that your overall gameplay is not good enough yet. So my suggestion is, don't play DT. Don't play it until you're immediately comfortable playing it, because you'll learn to spam quickly, and maybe get some decent DT scores, but you'll be useless as an overall player, without solid basics - which is nomod.
Source: experience, I started playing DT at your rank, then had to re-learn the game after I hit a point of no improvement.